Threads like these are always exciting on D.C.
If I have only one gun and it must be all things to me, make it a shotgun. Birdshot for hunting, buckshot for bigger hunting and/or short range self defense, and slugs for about anything you absolutely, positively, must mess-up severely on the first shot.
That said, I assume you have other weapons in your arsenal at this time? That maybe a factor.
Back to the one gun thing though... how many outdoor stores sell shotgun ammunition? How many sell 7.62 x 39mm? Where I live, about ten times as many sell shotgun ammunition. Food for thought.